The use of the illumination in keyboards of the membrane or lamina type is well known, particularly where the graphic layer, usually the top layer of a membrane switch is rendered to provide a dead front or blank appearance. The provision of a dead front or unidirectional mask for a membrane keyboard switch is generally accomplished by the choice of dark or black inks utilized in the printing of the graphic portions of the switch. This is typically done for esthetic reasons in order to provide a contrast between a given switch point or the colors chosen for the graphics. Dead front coverage may be utilized over the entire switch face or selectively in portions of the switch. When this is done, it is necessary to illuminate the graphics from the rear which is termed "back lighting" and is accomplished by making the membrane switch construction relatively transparent except for the dead front top layer which contains the graphics.
A variety of sources of illumination for back lighting have been employed. Included are incandescent bulbs which have the advantage of relatively low cost but the disadvantages of heat generation and relatively high energy consumption and a certain fragility caused by the need for a fine filament typical of incandescent bulbs. Also, fluorescent light sources have been employed with less of a consumption of energy but also having a certain fragility and a relatively higher cost. A third type of illumination now beginning to be used relative to display panels employs the phenomena of electroluminescence which shall be here termed EL. From an initial shortcoming of relatively high cost and limitations in regard to colors available, the EL concept has been extended first from displays made of glass to light sources, wherein the electroluminescing powders can be printed on the substrates such as polyester film by silk screening techniques. This capability opens the door to a wider use of EL as a light source and particularly where its use can be employed in a thin lamina structure such as a membrane keyboard. One of the problems with the employment of EL as a source of illumination is the fact that the effective life of the EL device is fairly limited, particularly wherein the electroluminescing powders must be driven hard in order to provide sufficient light for contrast purposes. In certain instances of commercial use of EL devices, the effective life has been on the order of 1200 hours or perhaps slightly more, which is considerably less than the effective life of the component or system which is served thereby.
In addition to limited life for EL devices, there is a considerably greater use of energy than with front lighting by incandescent bulbs.
A third aspect of the background of the invention relates to the fact that the desirability to limit access to the controls of electronic devices such as computer terminals, T.V. sets, control monitors, and the like. For example, it would be desirable if the horizontal, vertical and contrast controls of a television set might be sufficiently concealed or made "invisible" as to allow only an authorized person to adjust the picture on such receiver. The same holds for computer and telecommunication monitors and more particularly, to such CRT displays as are now used in aircraft and on industrial controls. Presently, most of these controls are visible either on the front of the display of the device or are located in an obvious position on the rear thereof and may be easily manipulated or tampered with or indeed, accidentally moved to the end dissatisfaction of the user.